The present invention relates to a liquid feed pump system, such as an oil pump, attached to an internal combustion engine.
An oil pump system is used to feed oil to oil supply portions of a crankshaft, dynamic valve mechanism, etc., of an internal combustion engine. In this pump system, an oil pump driven by the engine serves to compressively feed the oil, stored in an oil pan, to the oil supply portions through oil galleries and the like. When the engine stops, the oil in the oil pump, lowered in viscosity by heat, flows to the outside (oil pan) through small gaps at bearing portions or between joint surfaces of the pump housing. Thus, air may sometimes flow into the pump. If the engine is started in such a situation, the oil pump is first brought to a state (hereinafter referred to as "air-pump state") such that the air in the pump must be exhausted. Before the air is exhausted, the oil sucking capability of the pump is lowered extremely. Thus, it takes much time to start the oil being actually sucked up from the oil pan after the start of the engine. Such a situation is especially noticeable in the case of a gear-type oil pump. If flow resistance elements, such as an oil filter, check valve, etc., are located on the discharge side of the oil pump, therefore, the pump cannot discharge the air therein against the flow resistance from the resistance elements. In consequence, the oil cannot be fed to the individual oil supply portions, so that the engine may possibly suffer seizure or other trouble.